This refers to the used growing medium that is produced from the commercial mushroom growing industry. It can also be referred to as spent mushroom substrate, mushroom soil, recycled mushroom compost, or simply mushroom compost. Call it whatever you would like, it all means the same thing.

Mushroom compost is made of the following ingredients:

Bulking Agents - The most common bulking agents are straw, hay, horse manure, corn cobs, and cottonseed hulls. If you notice, most of these bulking ingredients act as a significant carbon source for the composting process.

Other - Several other ingredients are necessary when making mushroom compost, including water, gypsum (a source of calcium and sulfur used to regulate pH levels), and recycled leachate from previous piles of compost (helps to re-capture lost nutrients and prevent runoff into nearby waterways).

These ingredients are combined, and composted for approximately 3-4 weeks. Almost all commercial mushroom growers use thermal composting principles to create their growing medium. This means that they allow their compost to heat up to at least 150 degrees Fahrenheit, and then turn it as often as required for 3-4 weeks. This compost is then steam sterilized (killing some of the wonderful microbes), topped with peat moss (and lime), and used for production.

After production, mushroom compost is considered "spent", because it has already been used as a growing medium, and is no longer needed. Once again, it is steam sterilized, and sold. When you purchase spent mushroom compost, this is more than likely what you are getting. Some suppliers like to compost the spent growing medium again, however, most will offer it up for sale at this point in time.

As you may have heard, there is a lot of controversy surrounding the use of spent mushroom compost.

Is this controversy valid?

Yes, we believe it is. That being said, we still believe mushroom compost has a place in our fellow compost junkie's garden shed.

We would never deny the facts (refer to disadvantages listed above); mushroom compost is not perfect. It may contain some pesticide residues, high salts, and it may have fewer microbes than regular compost. However, these obstacles can be overcome.

For starters, we always recommend that you allow your spent compost to cure after it has been purchased/delivered. If you don't have time to let it cure, mix it, in a one to one ratio, with finished compost or garden soil, before applying it to your garden or lawn. This mixing accomplishes several different things: it helps to dilute some of the salts; it adds microbial life, and minimizes the affects of adding uncured compost to your garden.

Here is a great post by Laurie, a compost producer in Australia. In it she explains her technique for overcoming some of the disadvantages of using spent mushroom compost.

As for the pesticide residues, their effects will be minimized if you do two very important things: Inoculate your spent mushroom compost using compost tea or fully-cured compost, and allow it to cure for at least another month or two. When we have performed these two tasks, we have never noticed any ill-effects from pesticide residues. That being said, we rarely use spent mushroom compost in our edible gardens; we like to save our highest quality compost for this purpose.

It may also be of benefit to know that many commercial mushroom growers are now using Integrated Pest Management (IPM). This means that growers only apply pesticides as a last resort. We used IPM when we were working in the landscape industry, and can vouch for its effectiveness, when followed. Sadly, like in the landscape industry, you're going to encounter some growers who claim to follow IPM, but still tend to overuse pesticides.

In summary, spent mushroom compost will never be able to provide the same benefits as high quality compost; however, when you follow the above recommendations, it will definitely boost your garden's productivity.

Ornamental Gardens and Trees

After re-inoculating your mushroom compost, and allowing it to properly cure, use this compost as you would any other compost. For instance, we like to add one to two inches of it to each of our ornamental gardens, and around each of our trees, every other year. We usually alternate between adding compost one year, and composted mulch the next.

Since it's typically free of weed seeds, you can use mushroom compost as a mulch. If you're going to do this, we suggest applying it at least three inches thick.

Caution - Please avoid using spent mushroom compost on edibles, unless you are going to do the two important steps mentioned above. If possible, we recommend finding higher quality compost for your edibles. As the saying goes, it's better to be safe than sorry.

Above - An example of a lawn that we were in the middle of top-dressing with mushroom compost. The clients had asked us to apply it several inches thick (explains why our piles are so close together). This was prior to purchasing our power top-dresser, which made our jobs a lot more efficient.

Lawns

We've top-dressed 100+ lawns with spent mushroom compost, and each of them flourished. There were even some occasions when we didn't allow the compost to cure prior to spreading it; fortunately, the lawns still flourished.

One of the best things about spent mushroom compost is that it has a high amount of organic matter. If there is one thing we learned while caring for client's lawns, it was that they could ALL benefit from the addition of organic matter. This is especially true if the lawns were young (less than 10 years old).

If you plan on top-dressing your lawn with spent mushroom compost, please be sure you follow it with an application of compost tea. Also, if you're going to be reseeding your lawn, we suggest waiting two to three weeks after your top-dressing application. This will ensure you get the best germination.

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If so, you'll love our newsletter. If you sign up now, you will be given five free compost e-booklets, including one e-booklet on lawn care and compost?

All you have to do is click on the Lawn Care and Compost booklet to the right and use our Free Goodies page to sign-up.

Don't forget, if you're interested in finding a spent mushroom compost supplier near you, please take a moment and fill out our "Find a Compost Supplier Near Me" form at the bottom of our Buy Compost page.

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